1. Field of the Invention
The present invention relates to a DC to DC converter which is highly efficient for light loads and, more particularly, to a DC to DC converter having a current mode switching voltage regulator circuit and a feedback control path which regulates the switching frequency over broad current ranges based on the load without changing operational modes so that improved switching efficiency is made possible.
2. Brief Description of the Prior Art
A DC to DC converter is an apparatus that converts an input DC voltage into a constant regulated output DC voltage for application to a load. DC to DC converters of the type to which the present invention is directed employ a switching voltage regulator to convert the input DC voltage into the output DC voltage. The switching voltage regulator employs a switch, such as a power MOSFET, coupled either in series or in parallel with the load. The voltage applied to the load is regulated by controlling the ON and OFF time of the switch using a control circuit which varies the duty cycle applied to the switch based on the difference between the input DC voltage and the output DC voltage. The switching voltage regulator further employs inductive energy storage elements for converting the switched current pulses from the switch into a steady load current.
The duty cycle of the switch can be varied using a fixed frequency approach in which the frequency of the pulse stream is fixed and the ON or OFF time of each pulse is varied, or using a variable frequency approach in which the ON or OFF time of each pulse is fixed but the pulse stream frequency is varied. In either case, the output voltage is increased by increasing the ON time of the switch and is decreased by decreasing the ON time of the switch. A feedback circuit is used to accordingly vary the ON time of the switch so that a constant output voltage is maintained. The present invention may use either approach.
When the DC to DC converter is used in battery operated devices such as laptop and notebook computers and other hand-held electronic devices, the efficiency of the voltage regulator becomes critical to the effort to extend battery life. Typically, the efficiency of the voltage regulator decreases at low output current or low load conditions since the efficiency is generally a function of output current drawn by the load and since the losses associated with the operation of the switching voltage regulator become a larger percentage of the overall energy dissipation.
Numerous techniques have been proposed to improve the efficiency of voltage regulators for low load conditions. For example, in Unexamined Japanese Patent Application No. H4-42771, published Feb. 13, 1992, a DC to DC converter is disclosed which drives the duty cycle (PWM) control circuit intermittently during light load conditions so as to decrease dissipation caused by switching during such light load conditions. The output voltage is maintained in a predetermined range defined by upper and lower voltage thresholds when the output power of the DC to DC converter is below a low power threshold by turning off the switch control circuit so long as the output voltage level is above the lower voltage threshold. When the output capacitor discharges below the lower voltage threshold, the switch control circuit is again activated to cause normal duty cycle switching until the output DC voltage exceeds the upper voltage threshold. The switch control circuit is then deactivated until the output DC voltage again falls below the lower voltage threshold. This operation is repeated so long as the output power to the load remains below the low power threshold. Once the low power threshold is exceeded, normal duty cycle switching operation is resumed. Since less switching energy is dissipated for light loads, efficiency of the voltage regulator is improved.
FIG. 1 illustrates the step-down voltage regulator 10 of FIG. 2 of U.S. Pat. No. 5,481,178, which is characterized by a pair of synchronously switched MOSFETs Q1 and Q2 connected between the input DC voltage terminal V.sub.in and ground. Switching MOSFETs Q1 and Q2 are driven by driving elements 12 and 14, respectively, in a push-pull arrangement, and output power is provided to the output DC terminal via inductor L. The switching control circuit comprises comparators 16 and 18, constant OFF time one-shot 20, inverter 22, and logic gates NAND1 and AND1. A constant current source I1 and hysteretic comparator 24 are also provided for providing improved efficiency at low average current levels. During operation, the constant current source I1 and comparator 24 cause the push-pull switch comprising MOSFETs Q1 and Q2 to go into a so-called "sleep" mode in which both MOSFETs Q1 and Q2 are simultaneously OFF. The "sleep" mode is initiated when the feedback voltage V.sub.FB is greater than V.sub.REF, indicating that the output DC voltage V.sub.OUT is in excess of the regulated voltage and that the output can be maintained substantially at the regulated voltage by output capacitor C.sub.OUT. The overvoltage condition is intentionally induced at low average output currents by constant current source I1. In the sleep mode, other circuit elements may be deactivated as well. When it is detected by comparator 24 that V.sub.FB is less than V.sub.REF, the voltage regulator 10 wakes from the sleep mode and normal duty cycle switching resumes until the output capacitor is again overcharged and the "sleep" mode may be entered again.
The voltage regulator 10 of prior art FIG. 1 is generally undesirable because its design requires more current to be supplied to inductor L than is necessary to maintain the output DC voltage at the regulated voltage. Such over driving of the output capacitor C.sub.OUT leads to increased ripple voltage. A highly efficient voltage regulator is desired which minimizes the ripple voltage during low power conditions.
FIG. 2 illustrates the prior art voltage regulator 30 described in U.S. patent application Ser. No. 08/629,573, filed Apr. 9, 1996, and assigned to the present assignee. Voltage regulator 30 improves operating efficiency by providing a light load mode in which, rather than turning off the switching transistors for a "sleep" state, the rate of switching of the switching transistors is reduced in accordance with the load when a light load condition is detected. As illustrated, voltage regulator 30 includes synchronous switching transistors 32 and 34, comparator 36 for comparing the output DC voltage drop across R.sub.2 to a reference voltage V.sub.REF, switching control circuit 38, and feedback comparators 40 and 42. During operation, if the inductor current I.sub.L is detected by feedback comparator 40 to be greater than a threshold, comparator 40 outputs a level indicating that the output current is high. On the other hand, if the inductor current I.sub.L is detected by feedback comparator 42 to be less than another, lower, threshold, comparator 42 outputs a level indicating that output current is low. Comparator 36 monitors the voltage drop across R.sub.2, which is indicative of the output voltage V.sub.OUT and provides another control signal to switch control circuit 38. Switch control circuit 38 then turns ON switching transistor 32 when the control signal is received from comparator 36, and turns OFF switching transistor when the control signal is received from comparator 40. As a result, the duty cycle timing is adjusted in accordance with the rate of change of the inductor current, which is, in turn, related to the level of the load.
It is desired to provide an alternative voltage regulator in which the duty signal may be more accurately controlled for all load current levels. For example, it is desired that the same signal be used to determine the load condition and to reduce the switching frequency so that no offset error is introduced as in the voltage regulator of prior art FIG. 1. It is also desired to provide a technique to overcome the drawbacks of the voltage regulator of FIG. 2 by further reducing ripple voltage when the input DC voltage and output DC voltage are approximately the same. The present invention has been designed to address these needs.